The mono-alkynes Me3SiC≡CSiMe2(CH2CH=CH2) 7 and (CH2=CHCH2)Me2Si-C≡CSiMe2(CH2CH=CH2) 8, and the di-alkyne (CH2=CHCH2)Me2SiC≡C-C≡CSiMe2(CH2CH=CH2) 9, have been prepared and treated with Co2(CO)8 and Mo2(CO)4Cp2, respectively, to give the corresponding dimetalla-alkyne tetrahedral complexes. Two of the clusters have been structurally characterised: [Cp2Mo2(CO)4]Me3SiC≡CSiMe2(CH2CH=CH2), 11, crystallised in the space group C2/c with a = 15.058(3) Å, b = 12.474(2) Å, c = 29.128(6) Å, β = 100.12(3)°, V = 5386(2) Å3, Z = 8. [Co2(CO)6]2((CH2=CHCH2)Me2SiC≡C-C≡CSiMe2(CH2CH=CH2)), 13, crystallised in the space group C2/c with a = 12.604(3) Å, b = 15.447(3) Å, c = 18.382(4) Å, β = 107.20(3)°, V = 3418(12) Å3, Z = 4. Treatment of each of these clusters with HBF4 yielded the corresponding fluorosilane with concomitant formation of propene. It was shown that for [Co2(CO)6] (CH2=CHCH2)Me2SiC≡C-C≡CSiMe2(CH2CH=CH2) 15, reaction with only one equivalent of HBF4 yields the fluorosilane with loss of propene at the non-complexed alkynyl terminus. The relevance of these results to the possible intermediacy of a metal-stabilized silylium ion is discussed.